The present invention concerns a multi-focal spectacle lens with a dioptric power varying progressively between different zones of vision. Such spectacle lenses are referred to generally as progressive lenses.
That surface of the spectacle lens which effects the desired progressively varying course of the dioptric power of the lens is referred to as the progressive surface. The far-vision zone is located within its upper part and the near-vision zone is in its lower part. Between these two zones lies the progressive zone, in which the far-vision dioptric power of the lens progressively passes into the near-vision dioptric power. If a user of such a spectacle lens looks straight on and moves the line of sight downward to the bottom of the lens, the pre-established course of the dioptric powers for each inclination of the sight determines the distance of an object sharply focused. For each object distance, there is a corresponding inward movement of the line of sight and therefore of the person's eyes. The progressive surface of such a spectacle lens is characterized by a variation in dioptric power along the resulting accomodation-convergence path of the eye. This path is called the principal sight line. The principal sight line is a curve on the progressive surface of the lens, which is swung toward the nose. The principal sight line divides the lens surface into a nasal region and a temporal region.
A large number of different progressive lenses are known. In most of these lenses, such as those described for instance, in West German Provisional Patent (Auslegeschriften) Nos. 2,044,639, 2,336,708, and 2,439,127, and in West German published application (Offenlegungsschriften) Nos. 2,814,916 and 2,918,310, the progression line or the principal meridian is developed as a so-called umbilic line. This line in general does not follow the course of the principal sight line defined above. The umbilic development means that at each point of this line the surface curvatures are of the same value. In this way, the result is obtained that the surface astigmatism along this line has a value of zero.
In an article published in the journal Optica Acta, Vol. 10, No. 3, July 1963, pages 223/227, Minkwitz has proven that in a spectacle lens with an umbilic meridian line, the surface astigmatism in the direction transverse to this line changes at twice the rate of change of the average surface refractive power along the meridian line.
There has been no lack of effort to reduce to a tolerable magnitude the imaging error caused by use of an umbilic line, by special shaping of the progressive surface.
Thus, from West German Provisional Patent No. 2,044,639, it is known to develop the progressive surface in such manner that at intersection with horizontal planes there are produced, in each case, curves of intersection which have the shape of conical sections. This approach involves a certain compromise between astigmatism, focusing error and distortion, but this compromise does not satisfy higher requirements.
From West German Provisional Patent No. 2,336, 708, it is known to develop a progressive surface which is symmetrical to the umbilic meridian line in such a manner that the rate of change of the deformation of vertical or horizontal lines viewed through the spectacle lens is a small as possible, upon swinging the eye. This minimizing of the rate of change is one of various possibilities of improving the compatibility of a spectacle lens as compared with the previous prior art; however, its use alone does not lead to optimum results.
In West German published Application No. 2,918,310, it is proposed to form the progressive surface from two geometrically different surfaces. This approach yields the result that so-called skew distortion remains small on both sides of relatively wide progressive and near-vision zones. With this approach, which also involves symmetry with respect to an umbilic meridian line, the characteristic of the lens along the lines of intersection of the two geometrically different surfaces is disadvantageous, since discontinuity of the curvatures results in a discontinuity of astigmatism and focusing error.
In the spectacle lens known from West German Provisional Patent No. 2,439,137, oblique distortion in the lateral edge parts of the progressive zone and of the near-vision zone is to be caused to disappear by a special development of said parts; however, other imaging errors, particularly astigmatism, cannot be sufficiently compensated in this manner.
The spectacle lens in accordance with West German published Application No. 2,814,916 also involves symmetry with respect to an umbilic meridian line. With this lens a course of curvature of the progressive surface is established by formula, in order to obtain a large far-vision zone and a large near-vision zone. It will be understood that it is not possible in this way to keep imaging errors sufficiently small over the entire surface and still have an acceptable length of the progressive zone.
From West German published Application No. 2,610,203, it is known to develop the principal meridian non-umbilically in one part thereof. In this design approach, lines of the same average surface dioptric power are preferably horizontal and extend on both sides of the principal meridian up to the edge of the spectacle lens. Not all imaging errors can be optimally corrected by this aproach.
In a very early publication, namely, German Pat. No. 1,145,820, a progressive lens is disclosed in which the principal-curvature radii differ by a constant amount, at each point of the progressive surface; the surface astigmatism is therefore constant as to its amount. This approach results in less astigmatism in the lateral regions than in known solutions; but the technique uses a progressive surface of rotational symmetry, and there is insufficient correction of all imaging errors.
A spectacle lens generally has, in all regions outside its optical axis a prismatic power which becomes continuously stronger towards the edge of the lens. If, when viewing an object, both eyes look through regions of different prismatic power in the respective spectacle lenses, as for example, in the case of progressive lenses of known type, the directions of the eye-side lines of sight (i.e., line of sight directions on the eye side of the respective lenses) do not agree and therefore there is a horizontal and a vertical directional error.
In direct viewing, i.e., when looking at a fixed object, a horizontal directional error negatively influences the depth perception with the result, for example, that the points of a flat object no longer appear to lie in a plane.
In direct viewing, horizontal directional errors can be compensated by the vergency of the eyes within a relatively large range of tolerance. The compensating of vertical directional errors in direct viewing by fusional vertical vergency of the eyes, on the other hand, is possible only within narrow limits. For example, the binocular prismatic vertical tolerance for the vertical centering of two spectacle lenses is only 0.5 cm/m. The stronger the vertical directional error the more inconvenient the fusion becomes. If the directional errors are too great, the wearer of the spectacle lenses no longer has any impression of binocular vision. Either the wearer of the lenses will instinctively avoid looking through such regions of the spectacle lenses, or he will take into account only the visual impression from the left or from the right eye. Vertical directional errors can also lead to considerable orientation difficulty when reading, particularly when the eyes jump back to the beginning of a line of text.